This invention relates to the polymerization of olefin monomers in a liquid diluent.
Addition polymerizations are frequently carried out in a liquid which is a solvent for the resulting polymer. When high density (linear) ethylene polymers first became commercially available in the 1950""s this was the method used. It was soon discovered that a more efficient way to produce such polymers was to carry out the polymerization under slurry conditions. More specifically, the polymerization technique of choice became continuous slurry polymerization in a pipe loop reactor with the product being taken off by means of settling legs which operated on a batch principle to recover product. This technique has enjoyed international success with billions of pounds of ethylene polymers being so produced annually. With this success has come the desirability of building a smaller number of large reactors as opposed to a larger number of small reactors for a given plant capacity.
Settling legs, however, do present two problems. First, they represent the imposition of a xe2x80x9cbatchxe2x80x9d technique onto a basic continuous process. Each time a settling leg reaches the stage where it xe2x80x9cdumpsxe2x80x9d or xe2x80x9cfiresxe2x80x9d accumulated polymer slurry it causes an interference with the flow of slurry in the loop reactor upstream and the recovery system downstream. Also the valve mechanism essential to periodically seal off the settling leas from the reactor upstream and the recovery system downstream requires frequent maintenance due to the difficulty in maintaining a tight seal with the large diameter valves needed for sealing the legs.
Secondly, as reactors have gotten larger, logistic problems are presented by the settling legs. If a pipe diameter is doubled the volume of the reactor goes up four-fold. However, because of the valve mechanisms involved, the size of the settling legs cannot easily be increased further. Hence the number of legs required begins to exceed the physical space available.
In spite of these limitations, settling legs have continued to be employed where olefin polymers are formed as a slurry in a liquid diluent. This is because, unlike bulk slurry polymerizations (i.e. where the monomer is the diluent) where solids concentrations of better than 60 percent are routinely obtained, olefin polymer slurries in a diluent are generally limited to no more than 37 to 40 weight percent solids. Hence settling legs have been believed to be necessary to give a final slurry product at the exit to the settling legs of greater than 37-40 percent. This is because, as the name implies, settling occurs in the legs to thus increase the solids concentration of the slurry finally recovered as product slurry.
Another factor affecting maximum practical reactor solids is circulation velocity, with a higher velocity for a given reactor diameter allowing for higher solids since a limiting factor in the operation is reactor fouling due to polymer build up in the reactor.
It is an object of this invention to produce olefin polymers as a slurry in a liquid diluent utilizing continuous product slurry takeoff;
It is a further object of this invention to operate a slurry olefin polymerization process in a diluent at a reactor solids concentration high enough to make direct continuous product takeoff commercially viable;
It is a further object of this invention to operate a slurry olefin polymerization process in a diluent at higher circulation velocities.
It is yet a further object of this invention to operate a slurry olefin polymerization process in a diluent in a reaction zone of greater than 30,000 gallons; and
It is still yet a further object of this invention to provide a loop reactor apparatus having a capacity of greater than 30,000 gallons and having a continuous take off means.
In accordance with one aspect of this invention, an olefin polymerization process is carried out at a higher reactor solids concentration by means of continuous withdrawal of product slurry.
In accordance with another aspect of this invention, an olefin polymerization process is carried out by polymerizing at least one olefin monomer in a liquid diluent in the loop reaction zone to produce a fluid slurry comprising the liquid diluent and solid olefin polymer particles. The concentration of solid olefin polymer particles in the slurry in the zone is maintained at greater than 40 weight percent, based on the weight of polymer particles and the weight of the liquid diluent. The slurry, including a withdrawn liquid diluent and withdrawn solid polymer particles, is continuously withdrawn, as an intermediate product of the process. The intermediate product is exposed to a pressure drop in a flash zone, producing a second slurry and vaporized diluent. The vaporized diluent produced in said flash chamber is condensed by heat exchange, without compression.
In accordance with another aspect of this invention, a loop reactor olefin polymerization process is carried out by operating at a higher circulation velocity for a given reactor pipe diameter.
In accordance with another aspect of this invention, a loop polymerization apparatus is provided having an elongated hollow appendage at a downstream end of one of the longitudinal segments of the loop, the hollow appendage being in direct fluid communication with a heated flash line and thus being adapted for continuous removal of product slurry.